fix(email-renderer): export builder rows as valid MJML columns

packages/email-renderer/test/email.test.ts

@@ -57,6 +57,20 @@ const document: BlockDocument = {
 };
 
 describe("email renderer", () => {
+  test("builder column rows export without nesting mj-column inside mj-column", async () => {
+    for (const count of [1,2,3,4]) {
+      const draft=structuredClone(document);
+      const content=draft.root.slots.content[0]!.slots.content;
+      draft.root.slots.content[0]!.slots.content=[{id:"columns",type:"layout.columns",version:1,props:{},styles:{},slots:{columns:Array.from({length:count},(_,index)=>({id:`column-${index}`,type:"layout.column",version:1,props:{width:`${100/count}%`},styles:{},slots:{content:index===0?content:[]}}))}}];
+      const result=await renderEmail(draft,{mjml:true});
+      expect(result.ok).toBe(true);
+      if(!result.ok)continue;
+      const mjml=result.value.exports!.mjml!;
+      expect(mjml.match(/<mj-column\b/g)).toHaveLength(count);
+      expect(await validateMjml(mjml)).toEqual([]);
+      expect(mjml).toContain("Hello &lt;world&gt;");
+    }
+  });
   test("global brand logo survives HTML, AMP, and strict MJML export", async () => {
     const draft = structuredClone(document);
     draft.root.props.logoUrl = "https://example.com/logo.png";